1. Field of the Invention
The present invention relates to a servo control loop and, more particularly, to a servo control loop having optical feedback.
2. Description Relative to the Prior Art
An essential element in a variety of information processing systems of the type that includes optical scanning and/or tracking apparatus is a controllably positionable light beam deflecting mirror. In high speed scanning or tracking systems, the beam deflecting mirror must be capable of rapidly and accurately directing an incident light beam to a desired location. Various mirror type deflectors are known in the art. One type of deflecting mirror is electromagnetically driven and is commonly referred to as a galvanometer scanner. Systems of this type are capable of high performance but are expensive, complex, and can present hysteresis problems (see E. P. Grenda et al., "Closing the Loop on Galvo Scanners", Electro Optical Design, pp. 32-34, April, 1974). Another type of mirror deflector utilizes a mirror directly attached to a piezoelectric shear transducer that acts as a driver. The transducer driver is often referred to as a "bimorph" or a "bimorph bender". (See: J. J. Shaffer, et al., "Bender-Bimorph Scanner Analysis", Applied Optics, pp. 933-37, April, 1970; U.S. Pat. No. 3,544,201; U.S. Pat. No. 3,794,410; and U.S. Pat. No. 1,438,974). Bimorph scanners can give high performance, are simple in construction and low in cost. Because of these desirable characteristics, bimorph scanners have achieved general acceptance in the art. One disadvantage of a bimorph scanner of the type described above is that it is only capable of a very small deflection and tilt angle: 0.004 inches deflection and 1/2.degree. tilt angle are approximate numbers for a one-inch long bimorph bender. To increase mirror deflection and tilt angle, U.S. Pat. No. 3,981,566 discloses the use of a mechanical linkage coupling the bimorph bender to the deflection mirror that increases the range of motion of the mirror.
It is desirable to use a servo system to obtain precise control of the parameters of the deflection mirror i.e., position and velocity. In such a servo system a feedback signal indicative of the position or velocity of the deflecting mirror is derived. This feedback signal is compared with a reference signal indicative of the desired mirror position/velocity. A difference between the feedback signal and the reference signal produces an error signal indicating that the mirror is not operating as desired. The error signal, after amplification, is used to correct the mirror scan parameters. A problem in using such a servo system with a high-speed scanning system, however, is the difficulty in obtaining the feedback signal. For those applications where high-speed scanning is desired (e.g., in computer and video applications) the deflection mirror and any associated mechanical linkage is made quite small to reduce the effects of inertia. Because the scanning mirror and associated moving parts are extremely light, mechanical sensing of the mirror movement would "load" the system and thereby lower its fundamental resonant frequency. Lowering the fundamental resonant frequency of a mechanical scanner is tantamount to reducing the bandwidth of the scanner since little angular movement can be obtained in a mechanical scanner beyond its fundamental resonant frequency (since the response drops off at about 40 db/decade). Bandwidth reduction due to mechanical loading of small, lightweight scanning mirrors can be quite severe, easily reaching an order of magnitude or more.
U.S. Pat. No. 4,044,248 discloses apparatus for correcting the velocity of a scanning mirror wherein the feedback signal is obtained through an optical coupling technique. A light beam is directed onto the scanning mirror and imaged, upon passing through an optical grating, onto a photocell. The grating modulates the light beam passing therethrough so the photocell output signal has a frequency proportional to the velocity of mirror scan. From this output signal a feedback signal is derived that compensates for velocity errors of the scanning mirror. While this technique eliminates inertial loading of the scanning mirror, it provides only velocity correction of the scanning mirror. It is desirable for certain applications to be able to determine the direction of mirror scan and to maintain absolute mirror position throughout the scan.